1. Field of the Invention
The present invention relates to dielectric compositions used in thin-film build-up printed circuit boards (PCB), and more particularly to dielectric compositions having two steps of laminating temperatures.
2. Description of the Prior Art
Conventional dielectric compositions used in manufacturing build-up printed circuit boards are generally categorized into two groups: thermal curing resin coated copper (RCC) and UV curable dielectric material for forming micro via.
Generally speaking, conventional thermal curing resin coated copper material must be laminated by thermal laminator and vacuum thermal laminator at 170xcx9c180xc2x0 C., followed by etching of the positions of the via holes. Laser drilling is then carried out to form openings. Although the above process of resin coated copper material is the same as the process of using film type dielectric. via with adequate diameters and correct positioning of the openings cannot be achieved due to the limitations of etching and alignment accuracy when the diameter of via holes is under 4 mil (100 xcexcm).
For the dielectric material used to form micro-via under Ultra Violet light, via holes with a diameter under 5 mil (125 xcexcm) and good yield cannot be achieved because the material sensitive to UV radiation doesn""t have good reliability. In addition, UV curable liquid type dielectric material for build-up process makes it hard to control the thickness and build up layers. The heat resistance to solder is also poor due to the photosensitive groups in the material.
The object of the present: invention is to provide a dielectric composition having two laminating steps with different temperatures. The composition is also applicable in both additive and substractive process for build-up PCB process.
Another object of the present invention is to provide a film type dielectric composition which exhibits thickness accuracy within xc2x12% by precise coating.
The invention is achieved by providing a composition comprising a diamine curing agent having two different crosslinking reaction rates, an epoxy resin compound having at least two functional groups in each molecule and a suitable inorganic insulating powder. The composition obtained can be melted and adhered at different laminating temperatures and it is a suitable thin-film type dielectric composition for build-up boards.
The dielectric composition of the invention shows two different laminating temperatures at a medium temperature of 80xcx9c120xc2x0 C., and post curing at a high temperature of 160xcx9c180xc2x0 C. respectively in a thermal laminator or an oven. The dielectric composition is also applicable in both addition and substractive build-up substrate layers. Particularly, a laser drilling process is used to form via holes after the invented film type dielectric composition has been laminated on the inner substrate at a medium temperature and, then cured and baked at a high temperature. A higher density build-up board with 1xcx9c2 mil (25xcx9c50 xcexcm) diameter via holes can be obtained by using addition process on the invented film type dielectric. Meanwhile, the thin-film type dielectric material is capable of attaining films with a thickness accuracy within xc2x12% by precise coating process. Moreover, the impedance control of the build-up layer is easily obtained.
Having the characteristics mentioned above, the dielectric composition of the invention is applicable in the process of manufacturing high density printed circuit boards. The dielectric composition is firstly coated onto a plastic thin film, followed by a conventional procedure of dry-film transfer method which can transfer the film type dielectric insulating resin to a high density printed a circuit board at 80xcx9c120xc2x0 C. By contrast, the dielectric material from the conventional processes exhibits a lamination temperature higher than 150xc2x0 C. and cannot be transferred at a lower temperature.
Furthermore, a new type of high density printed circuit board can be obtained by plating copper type dielectric or by laminating copper foils on the surface of film type dielectric material.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.
The dielectric composition having two laminating steps with different temperatures disposed in the invention is comprised of: 10xcx9c55 wt % of a diamine curing agent, which contains asymmetric chemical groups with different reactivities toward epoxide groups; 45xcx9c95 wt % of a flame-retardant epoxy resin compound having at least two epoxy groups; and an inorganic insulating powder in an amount of 5xcx9c50% of the sum of the amine curing agent and the epoxy resin compound.
The chemical structure of the diamine curing agent in the present invention is shown below: 
wherein, R1 represents 
and R2 is alkyl of C1-10.
The epoxy resin compound suitable in the invention is diglycidyl ether, wherein n greater than 2, and the epoxy equivalent is preferably 150xcx9c4000. Its chemical formula is as follows: 
wherein, R represents 
wherein a is an integer of 0xcx9c4 and b is an integer of 0xcx9c4.
The reaction of epoxy resin and curing agent forms a crosslinking structure in the epoxy resin so that a greater molecular weight brings increased strength to the thin film. In addition, agents are added to increase both the strength and flame retardance of the film.
Examples of suitable flame retardants are halogen, nitride, melamine, Sb2O3, Sb2O5, Al(OH)3.3H2O, bromine epoxy resin and phosphide illustrated below and combinations thereof. 
The species of the epoxy resin and curing agent, reaction of the epoxy resin and toughener (control of reaction temperature and time) and the types of tougheners are all important factors of the formation of the dielectric composition of the invention. Because a suitable curing agent is a compound having asymmetric structure at two ends of a molecule. Thus, the end having a faster reactivity partly forms bonds with the ring-opened epoxy resin when the formula is at B-stage. When the thin film is baked to reach C-stage, the other end having a slower reactivity takes part in the reaction and forms a crosslinking structure, illustrating the function of the dielectric composition of the invention""s two laminating temperatures.